In recent years, there is a demand to reduce the drive loss of the automatic transmission of an automobile, in order to improve the fuel consumption of the automobile. For the purpose of hydraulic sealing, a seal ring is attached to the automatic transmission. However, the friction loss of the seal ring leads to the drive loss of the automatic transmission. Therefore, one important task is to reduce the friction of the seal ring. In addition, the capacity of an oil pump of the automatic transmission is a significant factor that causes the drive loss, and therefore there is a demand to reduce the amount of oil leakage from the seal ring to thereby reduce the capacity of the oil pump. To reduce the drive loss of the automatic transmission to thereby improve the fuel consumption of the automobile, it is necessary for the seal ring to have low-friction characteristics and low-leakage characteristics.
FIG. 1 shows the basic structure of a hydraulic circuit using a seal ring. The seal ring 1 is attached to a shaft groove (ring groove) 4 formed on the outer peripheral surface of a shaft 2 and disposed on opposite axial sides of a hydraulic passage 3. Hydraulic oil supplied from the hydraulic passage 3 is received by the pressure-receiving side-surface 11 and inner peripheral surface 12 of the seal ring. The outer peripheral surface 13 of the seal ring is in contact with the inner surface of a housing 5, and the contact side-surface 14 of the seal ring is in contact with a side surface of the shaft groove 4. The hydraulic pressure is thereby sealed. Generally, the shaft 2 is rotatable, and the housing 5 is stationary. However, a combination of a stationary shaft and a rotatable housing may be used.
A method generally used to reduce the friction (friction loss) of a seal ring is to reduce a pressurizing load for pressing the contact side-surface of the seal ring that serves as a principal sliding surface against the ring groove. More specifically, a seal ring having a cross-sectional shape that allows the pressure of supplied oil to act between the contact side-surface of the seal ring and the ring groove is used to reduce the pressurizing load by the action of a cancelling load.
Patent Literature 1 discloses a method of reducing a pressurizing load by using a seal ring including side surfaces forming a tapered shape having an axial width that decreases from an outer peripheral side toward an inner peripheral side. With this configuration, a cancelling load is generated between a ring groove and a side surface of the seal ring. The tapered shape formed by the side surfaces can significantly reduce the pressurizing load and is currently known as the shape of a seal ring that can minimize friction.
Patent Literature 2 discloses a seal ring including: peripherally spaced recessed sections (pockets) 6 formed at least on the inner peripheral side of a contact side-surface; and pillar sections 7 disposed between the recessed sections 6, as shown in FIG. 2(A). As shown in FIGS. 2(B) and 2(C), each recessed section 6 includes: a deepest inclined portion 51 formed such that the axial width (thickness) of the seal ring decreases in an inner peripheral direction; and converging portions 52 that are located on opposite peripheral sides of the deepest inclined portion 51 and converge toward the innermost peripheral points of adjacent pillar sections 7. In this configuration, when the seal ring rotates, the oil filling the recessed sections 6 is squeezed along inclined surfaces of converging portions 52, and a lift 60 is thereby generated. In addition, hydraulic pressure acts on the recessed sections 6 on the contact side-surface. This causes a pressing load reduction effect (cancelling pressure 61), and friction is thereby reduced. In the seal ring in Patent Literature 2, a side surface of the seal ring is in surface contact with the ring groove and slides thereon, as shown in FIG. 2(D). Therefore, no leakage passage is formed in a gap of the abutment joint of the seal ring, and low-leakage characteristics are thereby obtained.
In the seal ring in Patent Literature 1, the sliding contact between a side surface of the seal ring and the ring groove is line contact, and the circular sliding line is located on the gap of the abutment joint of the seal ring. This causes a problem in that the oil leaks from the gap of the abutment joint. Although the use of the recessed sections in Patent Literature 2 allows a reduction in friction, the degree of reduction is lower than that in the seal ring in Patent Literature 1. Therefore, there is a demand to further reduce the friction.